1. Field of the Invention
The present invention relates to a broadcast receiving device for displaying closed caption data and a method thereof. More particularly, the present invention relates to a broadcast receiving device for determining whether to display closed caption data according to the level of noise included in a composite video signal and a method thereof.
2. Description of the Related Art
Recently, some broadcast receiving devices have included a function for displaying closed caption data which displays caption information on one side of a display screen.
The closed caption data is caption data recorded in an area of a video signal. The function for displaying closed caption data comprises a function for decoding the closed caption data and then displaying the caption data on an area of the display screen. The closed caption data has been developed to help hearing-impaired people, but it is also useful for a variety of other situations such as spelling education for children, English education for foreigners, and for providing exact communication in a noisy environment such as when relaying sports. In the case of the National Television System Committee (NTSC) standards, the closed caption data is overlapped and received in line 21 of the vertical blanking interval (VBI). The closed caption data can be selected to be displayed according to user preference, unlike the open caption data.
FIG. 1 shows an exemplary waveform of a signal loaded in line 21 of the VBI in the NTSC standards. According to FIG. 1, the line 21 includes a color burst signal 10, a sine wave of 7 cycles (ClockRun-IN) 20, and a caption data signal 30. The caption data signal 30 consists of a beginning bit section of 1 bit and a letter display section of 2 bytes, respectively. Each byte of the letter display section includes clean data of 7 bits and a parity of 1 bit.
The broadcast receiving device slices a caption data signal according to a cycle of the sine wave 20. In this case, the broadcast receiving device uses a fixed slicing level. That is, after an input signal is AD(analog to digital)-converted into a digital signal, the digital signal is compared with the slicing level to determine 0 or 1 bit.
However, video tape or television broadcast signals, including for example, terrestrial waves, NTSC signals, CVBS signals, and composite signals are generally analog signals, so noise can be inserted according to the channel condition and tape condition.
FIG. 2 shows an exemplary waveform of line 21 of a signal inserted with noise. As shown in FIG. 2, noise can be inserted in a section of the sine wave 20 and a section of the caption data signal 30. In this case, a value of the digital signal to be compared with the slicing level becomes different such that caption fonts may be garbled.
In addition, as noise is inserted in the section of the sine wave 20 such that a cycle of caption detection becomes difficult, caption information can be shown on a different area of the display screen. Therefore, the overall image quality of broadcasting can become degraded.
Accordingly, a need exists for a system and method for determining whether to display closed caption data according to the level of noise included in a composite video signal.